1. Field of the Invention
The present invention relates to a method of measuring local similarities between prestacked 3D seismic trace cubes obtained from a volume of an underground zone, or after repetitive prospecting surveys (4D). A local coherence measurement gives in the first place the similarity of a seismic cube in relation to another one, while accounting for the local similarity within a single cube.
2. Description of the Prior Art
The concept of proper coherence is a relatively recent development. Until now, the issue was to develop a tool revealing the stratigraphic or structural changes (notably faults) from seismic measurements, and thus to obtain volume information on these changes. The foundation of all the methods developed for less than ten years defines a local dissimilarity from trace to trace.
A first algorithm described by: Bahorich, M., and Farmer, S. (1995), “The Coherence Cube”, The Leading Edge, 14, 10, 1053–1058, calculates the cross-correlation between each trace of a seismic cube with two in-line neighbors, with two CDP (common depth point) neighbors, then in combining the two results, after normalization the neighbor by the energy of the traces. The coherence is estimated only from three traces, which makes calculation very fast but not very robust if the data contains noise.
According to another algorithm described by Marfurt, K. J., t al. (1998), “3-D Seismic Attributes Using a Semblance-based Coherency Algorithm”, Geophysics, 63,1150–1165, the coherence calculation is based on a local semblance calculation involving more traces, which makes the result more robust to noise.
According to another algorithm described by Gersztenkorn, A., and Marfurt, K. J. (1999), “Eigenstructure based Coherence Computations as an Aid to 3-D Structural and Stratigraphic Mapping”, Geophysics, 64, 1468–1479, the coherence calculation is based on an expansion into eigenvalues: an analysis window defined in lines, CDP and time is extracted from the seismic cube, the seismic trace covariance matrix is formed and the largest eigenvalue of this matrix is calculated. The coherence value then corresponds to the ratio between this eigenvalue and the sum of all the eigenvalues of the covariance matrix, or trace of the covariance matrix, which is the total variance of the seismic traces of the analysis window.
All these approaches however have certain limits. In particular, a major limitation is that they are not applicable to the analysis of seismic multicube data.
In fact, the goal of these various coherence attributes is rather to map stratigraphic anomalies with the attributes not allowing evaluation of the coherence, either calendar (4D) or AVO (“Amplitude Versus Offset”). What is known is that there is to date no algorithm allowing to determine such attributes.
Generalized Principal-Component Analysis (GPCA) is a known tool allowing showing a possible information redundancy between groups of seismic attributes; GPCA can be suited for defining a local seismic data similarity measurement, from one cube to another, by analyzing a neighborhood around a current point, the notion of a group of attributes being related to the surveys in time or to for example the prestack seismic surveys.
This technique is implemented in the method described in French patent application 02/11,200 filed by the assignee, for compacting and filtering seismic events read on “multicube” seismic traces, with distribution of these events in families corresponding each to a particular physical meaning: iso-offset or iso-incidence angle data cube, elastic parameter cubes resulting from a joint stratigraphic inversion, etc., in order to extract information on the nature of the subsoil. This method comprises forming, by combination of the seismic variables, synthetic variables in much smaller number, obtained by construction of an orthogonal vectorial base in each one of the analysis sets consisting of the data of each family, hence formation of an orthonormal vectorial base describing these analysis sets, and use of this orthonormal vectorial base (new attributes) for filtering and describing said seismic events.